"Robert Wolcott" writes:

I measured the voltage across the bnc connector (disconnected from diode)
again and it was 5.015 volts. The diode was open in both directions with my
diode tester (Fluke 177). What usually causes these to fail?

They can just fail on their own. In that circuit, current is safely
limited by the 10K resistor. Not much other than excessive current
or excessive reverse voltage could damage a low speed photodiode.

Just install any cheap photodiode with a large enough active area and
see if it works. A $2 one from Digikey would likely work just fine.

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Thanks,
Bob


"Sam Goldwasser" wrote in message
...
"Robert Wolcott" writes:

I'll give that a try. This diode is part of a relative power meter on my
yag laser. The meter is currently not working and I'm not sure if it is
the
diode or the circuit. The circuit can be seen at:

http://oregonstate.edu/~wolcottr/Yag...0schematic.jpg

Tests I have run include:

Measured the voltage supplied to the diode via BNC connector and it is
5.0
volts

Is the input 5 VDC or 12 VDC? 5 VDC wouldn't peg a 1 mAFS meter since
there is a 10K current limiting resistor.

placed a 1k resistor across the BNC connector and the power meter was
pegged.

Sounds consistent with 12 VDC input and the circuit working properly.

Hooked up the photodiode to the circuit and shined a 5mw diode laser into
it
and there was no reading (direct illumination).

Do the tests above but sounds like it's bad.

Does this sound about right?

Yep. You can also test the photodiode on the diode check range of your
DMM. It should test like a silicon diode. I bet it tests open both ways.